High stability emitter follower

ABSTRACT

An emitter follower apparatus having a low impedance output which is independent of temperature by cancelling the voltage drifts due to base-emitter diode changes in two cascaded emitter followers.

United States Patent Pieter De Wit Baltimore, Md. 842,530

July 17, 1969 Apr. 6, 1971 Inventor Appl. No. Filed Patented Assignee by the Secretary of the Air Force HIGH STABILITY EMI'I'IER FOLLOWER The United States of America as represented- [56] References Cited UNITED STATES PATENTS 2,963,656 12/1960 Parris 330/17 3,100,876 8/1963 Schulz 330/17 Primary Examiner-Roy Lake Assistant Examiner-Lawrence J. Dahl Attorneys-Harry A. Herbert, Jr. and George Fine ABSTRACT: An emitter follower apparatus having a low impedance output which is independent of temperature by cancelling the voltage drifts due to base-emitter diode changes in two cascaded emitter followers.

Patented April 6, 1971 INVENTOR. fire)? DE Wu BY x HIGH STABILITY EMITTER FOLLOWER BACKGROUND OF THE INVENTION Generally, an emitter follower amplifier is a single-stage degenerative amplifier in which the output is taken from across the emitter resistor. This circuit is essentially an impedance matching device for matching a high impedance cir- SUMMARY OF THE INVENTION The present invention utilizes a complementary pair of transistors arranged in an emitter follower configuration to provide a low impedance output which is substantially inde pendent of temperature. The emitter follower circuit is biased to allow equal currents to flow through the transistors. Since one transistor is an NPN and the other is an PNP, the baseemitter voltage drops of the respective transistor cancel each other. Therefore, any temperature induced changes in the base-emitter junctions of these transistors will be in opposite directions and will cancel each other.

It is one object of the invention, therefore, to provide an im proved high stability emitter follower apparatus having a substantially constant output voltage over a large temperature,

variation. g g V It is another object of the invention to provide an improved emitter follower having high stability when operated over a wide temperature range.

It is another object of the invention to provide improved emitter follower having substantially the same voltage which is applied to its input at its output.

These and other advantages, features and objects of the invention will become more apparent from the following description taken in connection with the illustrative embodiment in the accompanying drawing, wherein the FIGURE is a schematic diagram of the high stability emitter follower.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown the schematic diagram of the high stability emitter follower utilizing an N PN and PNP transistor. The input voltage is applied to the base 11 of transistor 14 which is an NPN transistor at terminal 10. The collector 12 of transistor 14 which is coupled to terminal 15 receives the positive power supply voltage that is applied thereto. The emitter 13 of transistor 14 is coupled to ground by means of emitter resistor 16. The base 18 of transistor 21 which is a PNP transistor is directly connected to the emitter 13 of transistor 14. The positive power supply voltage at terminal 15 is coupled to emitter 19 of transistor 21 by means of emitter resistor 17. The collector 20 of transistor 21 is directly coupled to ground. The output voltage of the high stability emitter follower is taken from terminal 22 which is directly coupled to emitter 19 of transistor 21.

When a positive voltage is applied to terminal 10, the transistor 14 will begin to conduct. The voltage at the emitter 13 will begin to rise (go positive) and this voltage which is also applied to the base 18 of transistor 21 will cause transistor 21 to conduct. As transistor 21 begins to conduct the voltage at the emitter 19 decreases from substantially the positive power supply voltage which is applied at terminal 15 to a lower but positive value. When the initial circuit turn-0n perturbations have subsided and the transistors 14 and 21 are fully conducting, the base-emitter voltage drop of transistor 14 cancels the base-emitter drop of transistor 21. Therefore, the input voltage which is applied at terminal 10 is substantially equal to the output voltage that appears at terminal 22. It maybe noted that the base-emitter junctions of transistois 14, 21 will be affected in substantially the same manner by any increase 'or decrease in temperature thereby maintaining the corresponding cancellation of the respective base-emitter voltage dropsL- Therefore, for circuit applications where extremely stable operation over a temperature range is required, the high stability emitter follower which was described above will be found to be useful. Over the full military temperature range (-55 to l25 C.), the DC output drift is 2 millivolts. The following mathematical computation will more fully demonstrate the cancellation process which occurs within the circuit. The output of the first stage (the emitter 13 of transistor 14) is given by:

V =the voltage at the emitter 13 V,-,, the applied input voltage VE the base-emitter voltage drop of transistor 14.

The output of the second stage (transistor 21) which appears at terminal 22 is,

' where V thevoltage at emitter 19 v V the base-emitter voltage drop of transistor 21. Therefore, the base-emitter voltage drops of transistor 14, 21 cancel each other and the input voltage v,-,, equals the output voltage V,,,,,. The base-emitter diodes will track each other very closely when operated at substantially equal current levels. This may be accomplished by adjusting either emitter resistor 16 or 17.

The results of the stability test which was performed with this circuit are tabulated below. In the test circuit, the value of the emitter resistors 16, 17 was 50 K. ohms and the power supply voltage was +14 V DC. It may be noted that over the full temperature range the drift was 2 millivolts.

Temperature C It should be noted that this circuit provides a low impedance voltage source which is substantially independent of temperature.

lclaim:

1. An emitter follower apparatus for providing a stable output voltage over a wide temperature range comprising in combination'.

a first emitter follower having a first predetermined voltage drop, said first emitter follower having a first input and first output;

a second emitter follower having a second predetermined voltage drop, said second emitter follower having a second input and second output, said first and second emitter followers being operated at substantially equal current levels, said first predetermined voltage drop being substantially equal to but opposite to said second predetermined voltage drop, said first input receiving signal, said first output being directly connected to said second input, and second output providing an output signal.

2. An emitter follower apparatus as described in claim 1 wherein said first emitter follower comprises a transistor having a base, collector and emitter electrodes, said base electrode comprising said first input, said emitter electrode comprising said first output, said base and emitter electrodes forming a junction, and said junction having a first predetermined voltage drop.

3. An emitter follower apparatus as described in claim 1 wherein said second emitter follower comprises a transistor having a base, collector and emitter electrodes, said base electrode comprising said second input, said emitter electrode comprising said second output, said base and emitter electrodes forming a junction, and said junction having a second predetermined voltage drop.

5. An emitter follower apparatus as described in claim 3 wherein said transistor of said second emitter follower comprises a PNP device. 

1. An emitter follower apparatus for providing a stable output voltage over a wide temperature range comprising in combination: a first emitter follower having a first predetermined voltage drop, said first emitter follower having a first input and first output; a second emitter follower having a second predetermined voltage drop, said second emitter follower having a second input and second output, said first and second emitter followers being operated at substantially equal current levels, said first predetermined voltage drop being substantially equal to but opposite to said second predetermined voltage drop, said first input receiving signal, said first output being directly connected to said second input, and second output providing an output signal.
 2. An emitter follower apparatus as described in claim 1 wherein said first emitter follower comprises a transistor having a base, collector and emitter electrodes, said base electrode comprising said first input, said emitter electrode comprising said first output, said base and emitter electrodes forming a junction, and said junction having a first predetermined voltage drop.
 3. An emitter follower apparatus as described in claim 1 wherein said second emitter foLlower comprises a transistor having a base, collector and emitter electrodes, said base electrode comprising said second input, said emitter electrode comprising said second output, said base and emitter electrodes forming a junction, and said junction having a second predetermined voltage drop.
 4. An emitter follower apparatus as described in claim 2 wherein said transistor of said first emitter follower comprises an NPN device.
 5. An emitter follower apparatus as described in claim 3 wherein said transistor of said second emitter follower comprises a PNP device. 